This invention relates to radiation detectors, and more particularly to a scintillation detector which uses a photodetector to detect scintillation light produced in a scintillator in response to a radiation applied thereto and introduced by a light guide.
When a radiation is applied to a non-organic crystal such as NaI(Tl) formed by adding a small quantity of Tl to NaI or CsI(Eu) formed by adding a small quantity of Eu to CsI or to an organic crystal such as anthracene or stilbene, scintillation light having a certain wavelength is produced as a part of the relaxation phenomenon which occurs after the crystal excitation. Therefore, a radiation detector can be formed in which the scintillation light thus produced is applied to a photodetector such as a photomultiplier tube where it is converted into an electrical signal.
A scintillation detector operating on the above-described measurement principle has a time resolution of the order of 10.sup.-6 sec in the case of the non-organic crystal, and 10.sup.-9 sec in the case of the organic crystal, thus nearly reaching the resolution limit of the photodetector. Hence, such a scintillation detector can be used in an apparatus high in counting rate, and has a wide range of application to provide signals in a coincident counting method or in a time of flight method.
The above-described scintillation detector has a light guide to lead an output light beam of a light emitting material, which is a scintillator, to the photodetector while changing the direction of the light beam, or performing the expansion, reduction or division of the light beam. The light guide is generally made of glass or acrylic resin because they can be readily machined or are excellent in optical transmittance. The coupling surface of the scintillator and the light guide is generally a planar surface.
FIG. 1 is a diagram showing the coupling of a scintillator 10 and a light guide 12 in a prior art scintillation detector. The scintillator is relatively large in area, and small in thickness. One end of the scintillator is coupled through the light guide 12 to one end of a photodetector 14 which comprises, for instance, a photomultiplier tube.
The most important characteristic of the light guide 12 described above is its transmission efficiency. The transmission efficiency is affected by the optical transmittance of the light guide (i.e., the transparency for the light beam emitted from the scintillator 10), the reflection of light at the interfaces (which are the coupling surfaces of the scintillator 10, the light guide 12, and the photodetector 14), and the surface treatment of the light guide 12.
Of these factors, the reflection of light at the interfaces depends greatly on the refractive indices of the scintillator 10, the light guide 12 and a coupling agent (which is an adhesive used to couple the scintillator 10 to the light guide 12, and the light guide 12 to the photodetector 14). In the case of a scintillator high in refractive index such as that of Bi.sub.4 Ge.sub.3 O.sub.12 (BGO) having a refractive index of 2.15 which has been recently provided for measurement of gamma rays, since the refractive indices of the light guide and the coupling agent are generally 1.5 or less, the scintillator and the light guide or coupling agent are greatly different in refractive index. Hence, among light beams produced in the scintillator 10, the light reflected by the interface between the scintillator 10 and the coupling agent is increased in proportion while the light transmitted to the light guide 12 is decreased. Accordingly, the light transmitted to the light guide from the scintillator 10 is lowered in intensity, as a result of which the quantity of light reaching the photodetector 14 is decreased; that is, the radiation detector is lowered, for instance, in the sensitivity of detection.